Stereo evaluation apparatus with electronic ancillary devices for automatic correlation have been known for some time. For example, a stereo evaluation apparatus with an electronic correlator is described in "Photogrammetric Engineering", 1969, Volume 35, Number 8, pages 780 to 788. This stereo evaluation apparatus determines x-parallaxes in the evaluation procedure and derives signals therefrom which serve to position the elevation of the z-carriage of the evaluation apparatus which functions pursuant to the principle of mechanical projection.
The ancillary device aids in photoelectrically scanning the stereo images in order to obtain the signals for the correlator and includes flying spot scanners having scanning rasters which must be distorted during the scanning procedure for adaptation to the image geometry. In practice, this causes considerable problems since instabilities of the raster-form affect the correlation. In addition, since analog signals are correlated with each other which cannot be stored with this device, only image regions can be correlated with each other on the stereo images which have just been presented. It is not possible to obtain a correlation with additional aerial images, for example, for the purposes of point transformation or for making deformation measurements. The same disadvantages are associated with the apparatus described in United Kingdom Pat. No. 1,231,998 and U.S. Pat. No. 3,901,595.
In U.S. Pat. No. 3,883,251, a method for evaluating stereo images is described wherein the images are sequentially applied to a monocomparator and are photoelectrically scanned and the stored signals are subsequently correlated with each other. The scanning is achieved by displacement of the carriage in a direction beneath a linear array of detectors orientated with respect to the image in correspondence to the epipolar line geometry. This method does not offer the user a stereo view and is not suitable to facilitate for the operator the evaluation of aerial images on stereo evaluation apparatus in the context of a computer-supported semiautomatic mode of operation.
The known correlators utilized in combination with stereo plotters are relatively costly ancillary devices for whose implementation a great many physical changes have to be made on the evaluation apparatus. The foregoing notwithstanding, these correlators essentially simply permit only an automatic determination of x-parallaxes and therefore are suitable exclusively for the determination of contour interval lines or for making orthophotos. In these apparatus too, the stereo images must be manually orientated by the operator before the actual automatic evaluation.